This proposal is for a two-year project that expands the Seychelles Child Development Study a longitudinal cohort study following children and adolescents in the Republic of Seychelles who were exposed prenatally to MeHg through maternal consumption of a diet high in fish. It provides a unique opportunity to examine the controversial issue of whether ASD is related to MeHg exposure-an issue that has major public health implications in the United States. Recent pilot data from 445 adolescents participating in the SCDS suggested an adverse association between scores on the Social Reciprocity Scale (a teacher questionnaire used to screen for ASD symptoms) and prenatal MeHg exposure. The expected prevalence of ASD is 1:166, hence a very large cohort would be necessary to compare rates in the Seychelles and elsewhere. There are three adolescent cohorts available for study. The first, called the Main Cohort is being examined currently with funding from Grant 2-R01ES08442-05. The proposed project would expand the assessment of this cohort to include administration of the Social Communication Questionnaire (SCQ) a parental screening for ASD. The current assessment battery does not include measures of ASD. We would also screen parents of children from the two other cohorts using the SCQ (called the Pilot Cohort and the Third Cohort) who have not been examined in order to increase statistical power to detect ASD. The prenatal exposure to MeHg is known already for the Main Cohort (N=739) and Pilot Cohort (N=789) subjects. We expect to recall an additional 3,150 parents of Third Cohort children whose maternal hair samples have not yet been analyzed for Hg. The proposed project would permit expansion of the subject pool with known prenatal MeHg exposure to approximately 4,678 subjects. At the conclusion of the proposed project, we will estimate the association between prenatal MeHg exposure and SCQ scores. If warranted by the outcome, this newly characterized cohort could then be used for further study using standard diagnostic procedures to clarify the association between MeHg and ASD and to determine the prevalence of ASD in a population with known high exposure to MeHg from fish consumption during pregnancy. Public Health Significance Concern is growing among scientists and policy makers that there may be a link between exposure to methylmercury and ASD phenotype. There are no published human studies where prenatal exposure to methylmercury has been ascertained and offspring have been tested for ASD. The Seychelles Child Development Study provides a rare opportunity to test the hypothesis that there is an association between ASD and methylmercury exposure from fish consumption.